4:45 PM - 5:00 PM
[PCG18-18] K-Ar dating of Martian meteorites using a spot-by-spot laser ablation method
Keywords:K-Ar, LIBS, Mars
Radiometric dating of multiple geologic units by landers and rovers on Mars would be extremely valuable because it can give an absolute age to the stratigraphic (i.e., relative) age of different eras in the Mars history. Furthermore, in situ radiometric dating on the Mars surface would also be useful for selecting samples to return to Earth.
The potassium-argon (K-Ar) dating was carried out using the SAM instrument in the Curiosity rover by combining the Alpha Particle X-Ray Spectrometer and a quadrupole mass spectrometer (QMS)1,2,3. However, this method may have room for further improvements. For example, Vasconcelos et al.2 reported different ages from the two aliquots of the same rock. They interpreted that this is potentially because of incomplete Ar extraction from a rock, mineral sorting during the sample preprocessing, and incapability to measure K and Ar in exactly the same aliquot.
A new K-Ar dating method, which can conduct spot-by-spot age measurements, has been developed by different groups to resolve these problems (e.g., [4-6]). The system for K-Ar dating using a laser is a combination of laser-induced breakdown spectroscopy (LIBS) and mass spectrometry (MS). It has advantages that local analyses of a rock enable to obtain isochrons by laser ablation with a spot diameter of several hundred μm without the need to powder the sample. The K-Ar ages obtained using this LIBS-MS method have been reported for terrestrial gneiss and basalts5,7. However, samples from Mars, which are generally much poorer in K, were not analyzed yet. In this study, we investigate the feasibility of K-Ar isochron dating by conducting the spot analyses of a Martian meteorite.
We measured NWA1068, which is categorized as a shergottite. It was sliced and put in a vacuum chamber evacuated to pressure ~10-6 Pa. Then, 1000 laser pulses (266 nm, 25 mJ) were irradiated at each spot. Potassium concentration was measured with the emission line of K at 769 nm. In order to obtain K contents, an internal standard calibration curve was constructed using the intensity of the oxygen line at 777 nm. The amount of 40Ar was quantified by measuring the gas extracted by laser ablation with a QMS. The laser-ablation craters were observed under a microscope to measure the excavation volume. The mass of each laser pit was estimated from an assumed density of 2.7 g/cm3. The measurements were conducted at four different spots on the same shergottite sample by moving an XY stage.
The K2O concentrations and 40Ar amounts for the four measured spots were 0.23–0.43 wt.% and 1.6–2.4 ×10-14 mol, respectively. The slightly higher K concentrations compared to the bulk concentration of the shergottite (0.16 wt.% [8]) will be investigated by refining the K calibration model. Also, the masses of the ablation pits were 43–63 μg. The internal isochron age calculated from the four-spot data was 726±132 Ma. This age agrees with the K-Ar age of NWA 1068 reported (610 Ma [9]) within the error. This result strongly suggests the validity of the isochron dating based on this method. The amount of initial 40Ar, obtained as the intercept of the isochron, was <1.8×10-6 cm3STP/g. This value also agrees with the excess 40Ar reported by a previous study (1.32×10-6 cm3STP/g [10]) within the error range. These results support that our spot-by-spot analyses allow us to separate the contribution of trapped 40Ar from radiogenic 40Ar and improve the accuracy of the measured K-Ar ages, suggesting that the laser isochron method can yield accurate K-Ar ages of Martian rocks.
References: [1] Farley+. 2014 Science 343:1247166, [2] Vasconcelos+ 2016 JGR, 121:2176, [3] Martin+ 2017 JGR, 122:2803, [4] Cohen+ 2014 Geostand Geoanal Res, 38:421, [5] Cho+ 2016 PSS, 128:14, [6] Devismes+ 2016 Geostand Geoanal Res, 40:517, [7] Cho & Cohen 2018 RCMS, 32:1755, [8] Barrat+ 2002 Geochimica et Cosmochimica Acta, 66(19):3505, [9] Mathew+ 2003 EPSL, 214:27, [10] Bogard+ 2009 MaPS, 44(6):905
The potassium-argon (K-Ar) dating was carried out using the SAM instrument in the Curiosity rover by combining the Alpha Particle X-Ray Spectrometer and a quadrupole mass spectrometer (QMS)1,2,3. However, this method may have room for further improvements. For example, Vasconcelos et al.2 reported different ages from the two aliquots of the same rock. They interpreted that this is potentially because of incomplete Ar extraction from a rock, mineral sorting during the sample preprocessing, and incapability to measure K and Ar in exactly the same aliquot.
A new K-Ar dating method, which can conduct spot-by-spot age measurements, has been developed by different groups to resolve these problems (e.g., [4-6]). The system for K-Ar dating using a laser is a combination of laser-induced breakdown spectroscopy (LIBS) and mass spectrometry (MS). It has advantages that local analyses of a rock enable to obtain isochrons by laser ablation with a spot diameter of several hundred μm without the need to powder the sample. The K-Ar ages obtained using this LIBS-MS method have been reported for terrestrial gneiss and basalts5,7. However, samples from Mars, which are generally much poorer in K, were not analyzed yet. In this study, we investigate the feasibility of K-Ar isochron dating by conducting the spot analyses of a Martian meteorite.
We measured NWA1068, which is categorized as a shergottite. It was sliced and put in a vacuum chamber evacuated to pressure ~10-6 Pa. Then, 1000 laser pulses (266 nm, 25 mJ) were irradiated at each spot. Potassium concentration was measured with the emission line of K at 769 nm. In order to obtain K contents, an internal standard calibration curve was constructed using the intensity of the oxygen line at 777 nm. The amount of 40Ar was quantified by measuring the gas extracted by laser ablation with a QMS. The laser-ablation craters were observed under a microscope to measure the excavation volume. The mass of each laser pit was estimated from an assumed density of 2.7 g/cm3. The measurements were conducted at four different spots on the same shergottite sample by moving an XY stage.
The K2O concentrations and 40Ar amounts for the four measured spots were 0.23–0.43 wt.% and 1.6–2.4 ×10-14 mol, respectively. The slightly higher K concentrations compared to the bulk concentration of the shergottite (0.16 wt.% [8]) will be investigated by refining the K calibration model. Also, the masses of the ablation pits were 43–63 μg. The internal isochron age calculated from the four-spot data was 726±132 Ma. This age agrees with the K-Ar age of NWA 1068 reported (610 Ma [9]) within the error. This result strongly suggests the validity of the isochron dating based on this method. The amount of initial 40Ar, obtained as the intercept of the isochron, was <1.8×10-6 cm3STP/g. This value also agrees with the excess 40Ar reported by a previous study (1.32×10-6 cm3STP/g [10]) within the error range. These results support that our spot-by-spot analyses allow us to separate the contribution of trapped 40Ar from radiogenic 40Ar and improve the accuracy of the measured K-Ar ages, suggesting that the laser isochron method can yield accurate K-Ar ages of Martian rocks.
References: [1] Farley+. 2014 Science 343:1247166, [2] Vasconcelos+ 2016 JGR, 121:2176, [3] Martin+ 2017 JGR, 122:2803, [4] Cohen+ 2014 Geostand Geoanal Res, 38:421, [5] Cho+ 2016 PSS, 128:14, [6] Devismes+ 2016 Geostand Geoanal Res, 40:517, [7] Cho & Cohen 2018 RCMS, 32:1755, [8] Barrat+ 2002 Geochimica et Cosmochimica Acta, 66(19):3505, [9] Mathew+ 2003 EPSL, 214:27, [10] Bogard+ 2009 MaPS, 44(6):905